1. Field of the Invention
The present invention relates to an exposure apparatus for conducting a projection exposure of minute circuit patterns of integrated circuits such as ICs and LSIs on the surface of a wafer. More particularly, the invention is concerned with a projection exposure apparatus which makes use of light of different wavelengths for the exposure and for alignment.
2. Related Background Art
Exposure apparatus for projecting and printing minute circuit patterns of integrated circuits such as ICs and LSIs requires a projection optical system having a specifically high level of resolution. To cope with such a demand, it is a common practice to use light of wavelengths of 436 nm or 365 nm in the emission spectrum of a high-voltage mercury lamp. In order to precisely project and print the mask pattern on a wafer, the exposure apparatus employs an alignment optical system for detecting the relative position between the wafer and photomask. In order to enable an easy visual check of the state of alignment between the photomask and the wafer, the light source of the alignment optical system usually employs, for example, a He-Ne laser having wavelength of, for example, 632.8 nm.
It is also a common measure to employ in the alignment optical system a TTL system which makes use of the whole or a part of the projection optical system. In order to observe both the alignment mark on the wafer and the alignment mark on the mask simultaneously by the TTL system, it is necessary that the projection optical system can correct chromatic aberration both for the wavelength of the light used in the exposure and the wavelength of the light used in the alignment.
It is assumed here that light used for the exposure has a short wavelength 80 .sub.I of, for example, 436 nm, while the light used in the alignment has a long wavelength .lambda..sub.2 of, for example, 632.8 nm. A characteristic shown by a broken-line curve A in FIG. 6 is obtained as a result of chromatic aberration correction at these two wavelengths. In FIG. 6, the axis of abscissa represents the wavelength, while the axis of ordinate represents the axial chromatic aberration.
The actual light source has a certain band width of wavelength as indicated by .DELTA..lambda. in FIG. 6. Therefore, even if the chromatic aberration is completely absorbed at the wavelength .lambda..sub.I, a large chromatic aberration is caused at wavelengths which corresponds to the band width .DELTA..lambda. on both sides of the wavelength .lambda..sub.I. This large chromatic aberration seriously deteriorates the resolution of the projection optical system. In the two-color correction projection optical system having a characteristic as shown by curve A in FIG. 6, the spectrum of 436 nm from the high-voltage mercury lamp has a wavelength band width of about .+-.5 nm, so that the desired high level of resolution cannot be obtained unless the chromatic aberration is corrected materially completely over the wavelength band of 10 .mu..
To obviate this problem, an exposure apparatus has been proposed in which the exposure light source is capable of restricting the wavelength region into a narrow band, in order to obtain a high resolution even with a projection optical system after chromatic aberration for correction for two wavelengths. In such an exposure apparatus, a projection system with chromatic aberration corrected for two wavelengths as shown by solid-line curve B in FIG. 6 is used, and the exposure is conducted at a first wavelength .lambda..sub.1 (248.5 nm) while the alignment is effected at a second wavelength .lambda..sub.2. In this exposure device, however, only laser beam sources having restricted narrow wavelength bands are usable as the light sources for the exposure and alignment. Thus, this type of exposure apparatus cannot utilize high-voltage mercury lamps or halogen lamps which are commonly used as the exposure light sources. In addition, since the wavelength for the alignment is limited to only one wavelength value, there is a risk that the precision of detection of the alignment signal may be impaired depending on the condition of the photoresist on the wafer.